A display such as a liquid crystal display includes a backlight, which is a surface light source device for irradiating a display panel with light. The backlight includes: for example, an LED substrate on which LEDs are mounted as a light-emitting element; and a lens for diffusing light emitted from the LED. The LED substrate is generally strip-shaped, and LEDs are mounted at predetermined intervals in the longitudinal direction. Further, the lens is mounted on the LED substrate so as to cover the LEDs. The lens includes, for example, a light-entering area for light emitted from the LEDs to enter on the lower surface side, a light-emitting area for emitting the light entered from the light-entering area to the outside on the upper surface side, and projecting legs for mounting the lens on the substrate on the lower surface side. The lens is mounted on the LED substrate by bonding with an adhesive or the like at the end of the leg (Patent Literature 1).